Mill Computing, Inc. Forums The Mill Architecture Why not encode data types in metadata? Reply To: Why not encode data types in metadata?

Ivan Godard
Post count: 689


There are conceptual issues to what you suggest, and implementation ones.

The biggest problem is that you assume that that “add” can be disambiguated with a type. It can’t, because there are multiple flavors of “add” within each type. Floatint point adds have a rounding mode specification, while integer adds have an overflow behavior (saturate, fault, truncate, widen). To make that specification for any given op requires that the author, and later the generated code, assume a type for which the spec is meaningful. And if you make such a static assumption then you might as well put the type in the static opcode (addf vs addb vs addu vs …) and omit it in the (expensive) metadata.

Then there’s implementation. Real code has an immense number of silent type transformations, which would need to become explicit ops (to change the metadata), which would explode the code size, and add to the latency of a dataflow.

And there’s hardware issues. For area reasons op kinds are grouped so that related operations are done by related hardware, known as a Functional Unit or FO: ALU, FPU, LSU, etc. Instruction parsing breaks the binary apart to determine which FU of the several available executes the particular instruction, and then routes the rest of the instruction to that FU where further decode and execution happen.

However, the nature of what the ops are to do breaks apart naturally by type: integer operations go to the ALU and so on. In a static system, once the FU is decided the rest of decode and setup can be done in parallel with data argument fetch, using data paths that go only to the selected FU. In a type-dynamic system, the decoder could not choose the FU until the arguments had been fetched and the metadata examined, and then the lot including the args would be shipped to the FU. I think you can see that this would add at least a cycle to every operation; you probably cannot appreciate what the rat’s nest it would make of data routing, but take my word that it;s bad.

Meta-typing looks plausible for single-issue when performance is not an issue. It doesn’t work when there are large numbers of instructions being executed concurrently and performance matters.

So much for tutorial 🙂 I’m going to let this topic drop at this point.